I cut out and grinded a knive from an old saw blade.
I am thinking about hardening it in fire till it glows red and looses all magnetisation, and then quench it in vinegar, in order to get a grey-black appearance.
Can i do that, or is it not ok to do?
If not, can i etch it in boiling vinegar without loosing my heat treating?

Can you do it? Sure. Should you do it? Well, that depends on whether or not you want as good a performance as you can get out of the blade you're working so hard on. If you don't care how the blade performs then go ahead. Otherwise, quench in canola oil and then temper it properly.

If you're serious about making knives and not simply something that looks more or less like a knife then you might want to read the Stickies at the top of this forum, especially the one titled Before You Heat Treat your First Knife.You won't find everything you need in that post but it is a start. You need quite a bit more information on heat treating before you can count on creating a really good blade .

If you want to boil your blade in vinegar after the HT is finished it won't hurt the blade. On the other hand, if you were using a real blade steel like 1084 you wouldn't need to boil it, just dip it in ferric chloride and it will turn black in a few seconds ....

Thank you sooo much for taking your time, helping me.
I didn`t know that ferric chloride could turn the knive black too.
Now i`ll proceed to quench and heat treat it propperly.
Thank you for your help.

Ferric chloride might nor turn your mystery steel black but it will turn 1084 black. The problem with mystery steel is that you don't know what it is so it is hard to know how to heat treat it properly. or what will etch it the way you want, etc Let us know how your knife comes out .....

yeh ray nailed it on that one I have a friend who yes he used known carbon steels 1084 1095 ect....but he also does use things like old files saw blades that sorta thing he has been doing this for years even with his experience there is a good % that he forges out does everything then HT's it and finds out that it wont harden its a chance that he takes and understands...but if your serious about learning to make a knife your better off learning with KNOWN steels because say you use a file and it doesn't harden how do you know its a bad file or maybe your heat treat wasn't right....once you know how to heat treat then you know right away if its the file or not

You won't be able to 'polish' the scale off, you will need to sand it off to get to clean metal. As fuzzy said, you can use vinegar to remove a lot of it but sanding will be necessary in order to get to clean metal and to remove any pitting that may have happened in the heat treatment ...

First off do not quench in vinegar - it will only vaporize around the hot metal creating an gaseous layer that will not adequately cool the steel at a fast enough rate to get any kind of consistent hardening. Safest bet for most mystery steels is warmed canola oil - around 120-130 deg F. It is a little faster than other veg. oils. but not too fast. Once cooled and tempered in an oven wash well with soap/water before vinegar soak.

After an over-night in warmed vinegar (spot lamp will keep it warm just fine), take it to a wire wheel. This will take off most if not all of softened scale even in the pitted areas. Now you should be able to get a better idea of which direction you want to go with your final finishing. Depending on how well you ground out your bevels, you may be able to hand-sand from here to get your desired pre-sharpened finish.

A resoak in the vinegar will give you a nice dull gray patina that you can rub smooth with wetted 2000 grit wet/dry paper and very light finger pressure. Ferric will give you a much darker patina, but is much more aggressive - any scratches or pits will be enlarged and deeper, so just be aware of that. It will need to be "killed" with a baking soda/water solution or windex, then you can rub out same way with 2000 grit. Just remember any flaws will be accentuated using vinegar or Ferric so get what you don't want to see gone before you start any etching.